The detection and the identification of DNA or RNA can be effected by hybridisations with corresponding complementary nucleotide strands. These hybridisations take place with very high specificity and therefore have a high potential for the diagnosis and detection of diseases (Methods Enzymology 68, 373 [1979]).
One technique for carrying out such hybridisation experiments is the so-called Southern Blot method (J. Mol. Biol. 98, 503 [1975]), which, however, is rather complicated and has the further disadvantage that, as a rule, radioactive isotopes, e.g. .sup.32 P, are used in this technique. This is why many efforts have been made on the one hand to simplify the technique of the hybridisation methods, and on the other hand to replace the radioactivity by suitable non-radioactive reporter molecules.
One possibility for both simplifying the hybridisation technique and, at the same time, replacing the radioactivity is provided by fluorescent systems where an energy transfer takes place from a donor to an acceptor. Such energy-transfer systems were predicted by Forster (Ann. Phys. 2, 55 [1948]). At the same time, Forster (supra) produced a relation between the efficiency of energy transfer and the distance between donor and acceptor (so-called Forster equation). If the emission band of the donor overlaps the absorption band of the acceptor, energy can be transferred from the donor to the acceptor, and the efficiency of this energy transfer decreases with the 6th power of the distance between donor and acceptor. In other words, the intensity of energy transfer increases as the distance between donor and acceptor decreases.
Since then, energy-transfer measurements have become a useful tool for measuring distances both within and between various molecules (Stryer, Ann. Rev. Biochem. 47, 819 [1987])). Such measurements of distance via the efficiency of energy transfer between donor and acceptor molecules are suitable for immunoassays and for DNA Hybridisation assays (J. Biol. Chem. 251, 4172 [1976]; Anal. Biochem. 108, 176 [1980]; Clin. Chem. 29, 1582 [1983]; Chemiluminescent and Fluorescent Probes for DNA-Hybridisation Systems [1985]; Kingsburg, D. T. and Falkow S., Eds., 345-356, Academic Press, New York).